Porcelain enamel neutron absorbing material

ABSTRACT

A porcelain enamel composition as a neutron absorbing material can be prepared of a major proportion by weight of a cadmium compound and a minor proportion of compounds of boron, lithium and silicon. These compounds in the form of a porcelain enamel coating or layer on several alloys has been found to be particularly effective in enhancing the nuclear safety of equipment for use in the processing and storage of fissile material. The composition of the porcelain enamel coating can be tailored to match the coefficient of thermal expansion of the equipment to be coated and excellent coating adhesion can be achieved.

The U.S. government has rights in this invention pursuant to a contractbetween the U.S. Department of Energy and E.I. du Pont de Nemours andCompany.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to neutron absorbing material and, moreparticularly, to a porcelain enamel material having neutron absorbingproperties used for coating processing and storage equipment for use inthe nuclear industry.

2. Description of the the Prior Art

The design of chemical process equipment, such as piping, retention orreaction vessels, material handling and storage equipment, for nuclearindustrial operation requires a consideration of nuclear criticalitywhenever there is a possibility of the presence of fissile material inthe process stream. A criticality is an uncontrolled nuclear reactionresulting in an intense release of radiation and heat. In the absence ofshielding, a criticality presents a high potential for death of personsnearby (within 50 feet). The political or public impact of such an eventis even greater. Public reaction to such an event has resulted inextensive facility shutdowns and lawsuits. Even near criticalityincidents have resulted in similar public reaction. It is thereforeimperative that the nuclear industry do whatever is necessary to preventunplanned nuclear criticality.

Basically, the known physical and nuclear parameters are used to assurethat accumulations of fissile material are maintained subcritical bygeometry and mass control. In addition to these physical parameters inthe structural design of equipment, the use of neutron absorbingmaterials (also referred to as "neutron poisons") can increase the levelof criticality control associated with a piece of equipment.

One approach to providing a neutron absorbing protection is to fabricatethe process equipment of a material that includes one or more knownneutron poisons such as boron, cadmium, hafnium, and gadolinium. Forexample, stainless steel alloy that contains boron as a constituent hasbeen commonly used for the fabrication of reaction vessels, piping andstorage racks in the nuclear industry. The use of borated stainlesssteel, however, is very expensive because its fabrication requiresspecial alloy melts, castings and in some cases extensive machining.

There is also uncertainty about the corrosion properties of thespecially tailored alloys that may require extensive corrosion testing.For example, borated stainless steel equipment might have to be replacedsooner on the basis of a small acceptable wall loss for criticalitycontrol rather than because of a lack of physical integrity of theequipment.

Another approach, disclosed in U.S. Pat. No. 4,298,579 is a centrallyarranged neutron absorber rod in a tank containing a plutonium solution.The disclosure of this patent also suggests providing a tank wall withneutron absorbing material to be applied, in one instance, as a separatelayer in the form of a suitable enamel. The disclosure, however, doesnot provide any details concerning the proposed enamel nor is there anysuggestion concerning the form, composition or properties of the enamellayer.

Glasses containing boron, cadmium and other neutron absorbers are knownin the nuclear industry. See, for example Sun et al., "Neutron Absorbingand Transmitting Glasses", The Glass Industry, 1950, 31 (10) pp. 507-515and Melnick, et al., "Neutron-Absorbing Glass: CdO-SiO₂ -B₂ O₃ System",Journal of the American Ceramic Society, 1951, 34(3) pp. 82-86. Suchglasses while useful in a small scale laboratory setting may not meetthe physical integrity requirements for large scale processing andstorage equipment and are not suitable for application as porcelainenamel coatings.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved neutron absorbing material for use with processing and storageequipment where fissile material may be present. Another object of thisinvention is to provide a safe, versatile and cost-effective alternativeto the use of borated stainless steel and other neutron absorbingmethods for enhancing nuclear safety of nuclear industry process andstorage equipment. Another object is to provide a means for existingdesigns of processing and storage equipment to be made safer or toreduce the safe spacing of equipment with respect to nuclear criticalityfor increasing process or storage efficiency. Still another object ofthis invention is to provide a method of coating processing or storageequipment with a neutron absorbing material having a coefficient ofthermal expansion compatible with said equipment. These and otherobjects will become apparent to those skilled in the art from thefollowing specification and claims.

These and other objects are accomplished by providing a porcelain enamelcomposition as a neutron absorbing material. It has been found thatporcelain enamel neutron absorbing material can be prepared of a majorproportion by weight of a cadmium compound and a minor proportion ofcompounds of boron, lithium and silicon. These compounds in the form ofa porcelain enamel coating or layer on several alloys has been found tobe particularly effective in enhancing the nuclear safety of equipmentfor use in the processing and storage of fissile material. In addition,it has been found that the composition of the porcelain enamel coatingcan be tailored to match the coefficient of thermal expansion of theequipment to be coated and that excellent coating adhesion can beachieved. Also where extremely corrosive conditions exist, it has beenfound that the porcelain enamel layer can be adequately protected by acoating of polymerized tetrafluoroethylene, such as Teflon orFluoroshield .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows effect of porcelain enamel coating on the nuclear safety ofgeneric processing tanks at various concentrations of U-235.

FIG. 2 shows the effect of porcelain enamel coating on the allowablesafe spacing of processing tanks at a specific concentration of U-235.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention can be illustrated byreference to a method of coating of a type 304-L stainless steel vesselwith a layer of porcelain enamel. Type 304-L stainless steel is apreferred material of construction for many reaction vessels in nuclearmaterials processing and a coating composition was developed to bephysically compatible therewith. For the purposes of this specification,the primary physical parameter for compatibility of the materials beingthat the coefficients of thermal expansion should be close enough toprevent cracking of the porcelain enamel layer.

Accordingly an effective criticality safe reaction vessel can befabricated from type 304-L stainless steel by providing a 10 mil thicklayer of porcelain enamel on the steel surface. The porcelain enamel hasthe approximate composition:

    ______________________________________                                                CdO  69.5%                                                                    B.sub.2 O.sub.3                                                                    15.5%                                                                    Li.sub.2 O                                                                         2.5%                                                                     SiO.sub.2                                                                          12.0%                                                            ______________________________________                                    

Coating a stainless steel vessel with a porcelain layer of the aboveapproximate composition requires the basic steps of (a) porcelain enamelpreparation, (b) metal preparation, (c) spraying, and (d) firing. Anadditional step, providing a layer of a protective material, may berequired in some situations to protect the porcelain layer from exposurein certain corrosive conditions.

A. Porcelain Enamel Preparation

The porcelain enamel composition is milled using conventional techniquesin an aqueous suspension with the following mill addition:

    ______________________________________                                        Porcelain Enamel frit   1000                                                  Colloidal Silica        20                                                    Magnesium Sulfate       2.5                                                   Sodium Meta Silicate    1.25                                                  ______________________________________                                    

Milling is continued until all of the glass except a 1% residue passesthrough a 200 mesh screen (U.S. Standard). It has been found that aspecific gravity of 2.00 for the slurry (slip) is the most effective forspraying on the metal surface. However, due to the sensitive nature ofthe wet coating, the specific gravity may have to be adjusted based onthe ambient temperature and humidity. Spraying is the preferred methodof applying tee coating due to the density of the glass and its delicatesuspension in the slip.

B. Metal Preparation

As with conventional enameling practices, preparation of the metalsurfaces to be coated is essential for adequate adhesion of the enamelcoating. Grease and stain should be removed. Although any suitable metalcleaning technique can be used, sandblasting or etching are thepreferred surface preparation techniques. Surface irregularities such aswelds should be ground smooth prior to sandblasting or etching. Aftersurface preparation is complete, scratching or gouging the metal surfaceshould be avoided, because if has been found that such surfaceindentations can cause spalling.

C. Spraying the Prepared Metal Surface

It is desirable to apply sufficient glass slip or slurry on the metalsurface to achieve an enamel thickness of at least 10 mils with apractical upper limit of about 20 mils. It has been found that sprayingis the preferred method of application and that the enamel must besprayed in a fine spray. However, the sprayed slip tends to run or slideon the metal surface when the wet thickness exceeds 4 or 5 mils. Bycareful application techniques, it is possible to apply 6 to 8 mils ofenamel prior to each firing. It has been found that multiple passes withthe spray gun, laying one coat over the other and careful attention tothe moisture content of the slip will permit application of an optimumthickness of enamel for each firing cycle. Thus two coatings and firingswill achieve the desired porcelain enamel thickness of at least 10 mils.

D. Firing the Enamel Coatings

The enamel coating (bisque) on the metal surface should be permitted todry before firing. The firing parameters will vary depending on theshape and surface area being fired. By way of example, a 4×4×1/4 inchsample plate was satisfactorily fired at 1320° F. for 8 minutes. Also a4'×6"×1/4" tubular member was satisfactorily fired at 1100° F. byalternately firing for 8 minutes with 2 minutes removed for a total of100 minutes. After firing the initial layer, defects may be ground offor repaired, followed by respraying and a second firing to reach atleast a 10 mil porcelain layer. Normally with proper firing, theporcelain surface should have a good gloss, however, slight overfiringwhich yields a lower luster finish is not detrimental. Those skilled inthe art will recognize that testing different firing cycles should bedone to achieve an optimum porcelain layer on the metal surface for aparticular application.

In addition to type 304-L stainless steel, the porcelain enamel coatinghas been successfully applied to the following trademarked alloys:

    ______________________________________                                        HASTELLOY C-22    CARPENTER 20-Cb3                                            HASTELLOY C-276   INCOLOY 825                                                 HASTELLOY B-2     INCONEL 600                                                 HASTELLOY G-3     INCONEL 625                                                 HASTELLOY G-30    INCONEL 690                                                 MILD STEEL 1020                                                               ______________________________________                                    

The effectiveness of the porcelain enamel coating having neutronabsorbing properties for use or processing or storage equipment wasdetermined by subjecting porcelain-coated stainless steel components toa variety of tests as shown in the following examples. An additionalprotective step will be described in connection with the chemicalintegrity tests, Example III.

EXAMPLE I Mechanical Impact Test

Type 304-L stainless steel sample plates and 6" diameter cylinders werecoated with porcelain enamel of the previously described composition toan average thickness of 10 mils. The coated plates and cylinders wereimpacted with a 5/16" radius sphere at forces of up to 72 in-lb. Thesamples incurred only localized damage. The porcelain enamel in an areaapproximately 0.25" in diameter at the point of impact was pulverizedwith little or no spalling of surrounding material. Increased impactforce resulted in little or no change in the extent of damage. Theexposure of the stainless steel substrate was limited to the points ofimpact and did not significantly affect the overall integrity of thecoating.

EXAMPLE II Thermal Shock Tests

Three stainless steel cylinders (6"×6"×1/4") coated with the preferredporcelain enamel (average thickness--10 mils) composition were heated to150°, 200°, and 250° C., then rapidly quenched with water at eachtemperature. In the first test, ambient water was sprayed onto theoutside (coated) surface of the hot cylinders. No damage was observedafter 45 tests. Next, in order to quench from the inside, the hotcylinders were filled with ambient temperature water. In 55 tests,covering the three temperatures, spalling occurred on only one cylinder.This occurred on an edge and affected an area only 0.25" wide by 1.25"long.

Two cylinders were cycled 50 times between 50° C. and 155° C. using anoven and fan. No visual physical damage to the coating was observed.

It was concluded from the tests of Examples I and II that accidentalthermal shock and mechanical impact will not significantly affect theintegrity of the porcelain coating for the purpose of neutronabsorption.

EXAMPLE III Chemical Integrity Tests

A. Type 304-L stainless steel plates were coated with the preferredporcelain enamel composition to a thickness of about 10 mils. Standardleaching tests were conducted on the porcelain enamel samples with 51%nitric acid, nitric acid vapor, and deionized water. All were tested at90° C. for 28 days. The tests using deionized water showed very littleloss of cadmium or boron to the leachant. However tests with nitric acidand nitric acid vapor showed significant loss of cadmium and boron tosolution and separation from the stainless steel substrate. Becausethese were severe overtest conditions another more realistic test wasdesigned to assess the consequences of short term periodic contact withnitric acid.

B. An apparatus was set up to drop nitric acid onto a heated porcelainenamel sample plate so that each droplet would evaporate before the nextwas applied. This was intended to simulate a small leak. When the platewas held at 110° C., 50 drops resulted in localized damage approximately0.5 inch in diameter and 0.003 inch deep. At 90° C. 0.002 inch was lost.After these tests it was apparent that some means of protecting theenamel from chemical attack would be necessary to ensure that thecadmium and boron remain in place at all times.

C. A Fluoroshield (a proprietary polymerized tetrafluoroethyleneprotected by trademark) coating was applied over the porcelain enamel toprotect it from the nitric acid. Sample plates were coated and tested todetermine if the Fluoroshield provided sufficient protection from theacid. The Fluoroshield layer adhered very well to the enamel and nodamage was detected after chemical testing.

EXAMPLE IV Nuclear Safety Analysis and Tests

The results of nuclear criticality analysis (Monte Carlo calculations)are best illustrated by reference to FIGS. 1 and 2. FIG. 1 shows theeffect of lowering the criticality factor (K_(eff)) in genericprocessing tanks by the porcelain enamel coating at variousconcentrations of U-235. FIG. 2 shows the effect of the porcelain enamelcoating on the allowable safe spacing (at K_(eff) <0.95) of generictanks at a specific concentration of U-235. The criticality factor,K_(eff), is a measure of neutron multiplication in a fissile system. AK_(eff) of 1.0 is the point of nuclear criticality, above that point thenuclear reaction accelerates uncontrollably. The safe limit for K_(eff)has been defined as 0.95.

It is apparent, from FIGS. 1 and 2, that use of the neutron absorbingporcelain enamel on nuclear processing equipment can make that equIpmentsignificantly safer with respect to prevention of nuclear criticality.FIG. 1 shows that a generic tank which is unsafe for a wide range ofU-235 concentrations can be made safe for all U-235 concentrations whenthe coating is used. FIG. 2 shows that the allowable safe spacingbetween generic tanks at a U-235 concentration of 500 g/liter can bereduced by approximately 45% if the neutron absorbing porcelain enamelis used.

Further Monte Carlo calculations for specific processing vessels showedthat with the porcelain enamel, the vessels are completely safe underall credible normal or upset process conditions, whereas without theporcelain enamel, access to the vessels must be limited in order forthem to be safe. This data has been confirmed by neutron attenuationexperiments. The porcelain enamel coated samples met or exceededexisting neutron attenuation criteria. When comparing the nuclear safetyof porcelain enamel coated versus borated steel vessels the coated aremuch safer, especially when considering allowable wall corrosion loss.(As the wall corrodes away in the borated steel vessel, neutronabsorbing material is lost.)

The foregoing tests and examples are intended as illustrative and not tolimit the invention, which is intended as only limited as indicated inthe appended claims.

What is claimed is:
 1. A neutron absorbing composite comprising aporcelain enamel coating adhered to a chemical reaction vesselcontaining a process stream of fissile material, said coating consistingessentially, by weight, of a major proportion of oxide of a cadmiumcompound and minor proportions of oxides of a boron compound, a lithiumcompound, and a silicon compound in the form of a porcelain enamelcoating, said coating applied to said vessel in a thickness sufficientto prevent criticality.
 2. The composite of claim 1, wherein saidcomposite is a homogeneous coating of said porcelain enamel on saidvessel, said vessel made of a metal alloy, said coating consistingessentially of about 69 wt. % CdO, 16 wt. % B₂ O₃, 3 wt. % Li₂ O and 12wt. % SiO₂, said thickness of said coating being at least approximtely10 mils.
 3. The composite of claim 1, wherein said porcelain enamelcoating is protected by a coating of a polymerized tetrafluoroethylene.4. The method of making a chemical reaction vessel for the processing ofa stream of fissile nuclear naterials having nuclear critically safecharacteristics which comprises coating said vessel with a porcelainenamel having neutron absorbing properties and consisting essentially ofa major proportion, by weight, of a cadmium compound and a minorproportion, by weight, of a boron compound, a lithium compound, and asilicon compound, said coating in a thickness sufficient to preventcriticality.
 5. The method of claim 4 wherein said vessel is fabricatedof a metal alloy.
 6. The method of claim 4, wherein said porcelainenamel coating is protected by a coating of a polymerizedtetrafluoroethylene.
 7. The method of claim 5, wherein said metal alloyis stainless steel.